Also 'Tavi. This wiki engine was motivated by SunirShah's KeptPages idea, as well as Tavi:TaviCategories.
Available under the GPL, 'Tavi runs under PHP4 and uses MySQL to store the PageDatabase. It supports templates for the user interface, macro extensibility, etc. Available at http://tavi.sourceforge.net/. A feature list is at http://tavi.sourceforge.net/TaviFeatures.